NEW YORK — @BarackObama is on Twitter. So is @MittRomney. And so are all the voters following the 2012 presidential contest, whether they know it or not.

Candidates, strategists, journalists and political junkies have flocked to Twitter, the social networking hub where information from the mundane to the momentous is shared through 140-character microbursts known as tweets.

While relatively few voters are on Twitter — a study by the Pew Research Center found that about 13 percent of American adults have joined the site — it’s become an essential tool for campaigns to test-drive themes and make news with a group of politically wired “influencers” who process and share those messages with the broader world.

Put simply: When a voter is exposed to any information related to the presidential contest, chances are it’s been through the Twitter filter first.

“The subset of people on Twitter may be relatively small, but it’s a politically engaged audience whose influence extends both online and off,” said Heather LaMarre, a University of Minnesota communications professor who studies social media. “It’s not the direct message that has the biggest influence on people — it’s the indirect message.”

No one believes the campaign will be won or lost on Twitter; it’s just one slice of an enormous communication effort the presidential campaigns are waging in cyberspace. But with a well-timed 140-character blast, candidates influence coverage, respond to charges or reinforce talking points.

This, of course, is not the first time technology has changed the way campaigns are conducted. Radio, TV and the Internet all prompted campaigns to adapt, giving both more avenues to reach voters and more control of their message. But radio and television are top-down mediums at heart — from the broadcaster to the public. Never before has a grass-roots technology like Twitter given both voice and power to millions, and given candidates a real-time way to monitor the effects of their messages and recalibrate on the fly.

And that means an ever-changing campaign narrative for 2012.

Four years ago, Twitter was in relative infancy and just 1.8 million tweets were sent on Election Day 2008. Now, Twitter gets that many approximately every eight minutes.

Mr. Obama’s 2012 State of the Union address drew 800,000 tweets, Twitter said. And tweets mentioning Rick Santorum jumped from 10-20 per minute to more than 2,500 tweets per minute when news broke that he was suspending his bid for the Republican presidential nomination.

Both the Obama and Romney campaigns have actively embraced Twitter, using it to communicate directly with supporters and, more importantly, drive the political conversation in a way that reaches far beyond the site. They’re also mindful of the hazards of Twitter, designating war room staffers to monitor the site for problems to address or gaffes from their rivals to exploit.

“Our team understands that the most important issues in this campaign are jobs and the economy, not the Twitter controversy of the day,” Romney spokesman Ryan Williams said. “But we need to be on top of everything and monitor every aspect of this race. Twitter helps us keep our finger on the pulse of the fast moving pace of new media.”